In general, groundwork for civil engineering projects, construction works and the like involves work for preventing ground subsidence so that structures to be constructed on the ground can be structurally sturdily built. For such work, drilling operations are carried out for installing piles or beams into the ground or rock.
As a widely known apparatus for drilling a hole in the ground, FIG. 1(a) shows an apparatus for drilling a hole in the ground for installing a pile or the like which supports all the structures to be built mainly on the ground. Such an apparatus generally comprises a crawler travel device (100), a support arm (101) propped vertically by the crawler travel device, a drill machine or an auger machine (103) installed at the upper part of the support arm (101) for moving up and down, a rod (104) assembled at the auger machine (103) in multi-stages depending on the depth of a hole to be drilled, a hammer (105) attached to the bottom of the rod (104) to provide the power, or rotational force and impact, for drilling a hole in the ground, and a drill bit (106) installed at the hammer (105) for actually drilling a hole in the ground with the power provided by the hammer (105). The hammer (105) has a variety of sizes depending on the diameter of a hole to drilled, and though one drill bit (106) is normally installed at the bottom of the hammer (105), two or more hammers (105) and drill bits (106) can be mounted to drill a large diameter hole.
Recently, the increased large-scale civil engineering projects and construction works such as long bridges, highways, high-speed railroads, mega skyscrapers and the like require more large diameter holes to be drilled than ever. Thus, there is a great need for a technique for drilling a hole, which can maintain a high level of hole verticality and enhance work efficiency even when drilling a large diameter hole.
In order to meet such a need, various methods for drilling a large diameter hole in the ground have been proposed.
For example, Korean Patent Registration No. 10-0145495 (Date of Registration: Apr. 30, 1998) “Method and Device for Digging Ground by Combining Multi Hammer Auger Machine and Single Hammer Auger Machine” discloses an apparatus comprising a multi-hammer drill (210) and a single-hammer drill (220) concentrically coupled to each other, as shown in FIG. 2. According to the patent, the single-hammer drill (220) at the center is lowered to drill a small diameter hole using a drill bit (220a) of the single-hammer drill (220) as shown in FIG. 2(a), followed by enlarging the hole using a drill bit (210a) of the multi-hammer drill (210) installed outside the single-hammer drill (220). However, this technique requires two driving apparatus for separately controlling the operations of the single-hammer drill (220) at the center and the outside multi-hammer drill (210), thereby increasing the system complexity of the apparatus. In addition, in the technique described above, the multi-hammer drill (210) needs to remain stationary whenever the single-hammer drill (220) operates, and the single-hammer drill (220) should remain immobilized whenever the multi-hammer drill (210) operates. To this end, it is necessary to have a bearing structure between the multi-hammer drill (210) and the single hammer drill (220), which not only endures the impact caused by vibrations during the drilling and the impact from various dust and debris generated during the drilling process but also allows relative rotation and up-and-down movement relative to each other, though the above patent publication does not disclose it in the detailed description thereof. However, since such a bearing structure is hardly possible to manufacture because it demands extremely high precision, and if possible, the price is almost prohibitive, it is very difficult to actually implement such a technique.
Thus, it is rather common to use a single-hammer drill or a multi-hammer drill alone in turn depending on a drilling operation required, as described in the patent publication of Korean Patent Registration No. 10-0683021 (Date of Registration: Feb. 8, 2007) “Large-Caliber Excavator for Excavating Ground and Method thereof”. As shown in FIG. 3 attached herein, a single-hammer drill of small diameter is used to first drill a pilot hole (330) having a small diameter of w and then is lifted up, so that a multi-hammer drill (320) of large diameter is used to enlarge the hole to a large diameter of W(>w). However, such a technique has a drawback in that it needs substantially increased time to drill a deep hole. In addition, it may be very difficult to proceed the drilling if the rock under the ground collapses. In order to reach a deep location under the ground (such as 10 m below ground) using the single-hammer drill and the multi-hammer drill, the rods (104) need to be assembled in multi-stages (for instance, four rods of a length of 3 m). Accordingly, the single-hammer drill is first assembled at the bottom of the rod (104) to drill a pilot hole to a depth of 3 m, and then another rod is assembled at the top of the rod (104) to extend the length thereof. Thereafter, the single-hammer drill at the bottom of the extended rod (104) is used to drill the pilot hole (330) to another 3 m, thereby making a hole drilled to a depth of 6 m. Such a process is repeated until the hole reaches a desired depth. After the pilot hole (330) of the desired depth is completed through the above process, the rods (104) are disassembled again to make it short, and then the multi-hammer drill is assembled at the bottom of the rod and is lowered to enlarge the hole. Similarly, another rod(s) of a length of 3 m is (are) assembled to extend the total length of the rods so as to enlarge the hole depending on the depth as in the case of the single-hammer drill. Such a process is repeated to enlarge the hole to the desired depth. According to this method, for instance, in order to drill a hole of a depth of 10 m, the process of assembling the rods of a length of 3 m is repeated eight times (four times×two hammer drills). Thus, there exist problems in that the drilling operation slows down and the cost thereof increases because of the time and labor required for each assembly and disassembly of the rods, thereby increasing the chance of machine malfunction caused by errors during the assembly, and making it difficult to maintain the straightness of the hole to be drilled.
Therefore, there still exists a need for a technique for drilling a large diameter hole in the ground, which can not only precisely maintain the verticality of a large diameter hole during the drilling of the hole but also cut down the time required for drilling, and can facilitate the drilling operation.